1. Field of the Invention
The invention relates to a control apparatus for a vehicular power transmitting apparatus that performs slip control of a friction clutch.
2. Description of the Related Art
A friction clutch that is provided in a power transmitting path between an engine and driving wheels is well known. Examples include friction clutches such as a forward clutch (i.e., a starting clutch or take-off clutch) and lock-up clutch described in Japanese Patent Application Publication No. 2006-226333 (JP-A-2006-226333), Japanese Patent Application Publication No. 2005-351357 (JP-A-2005-351357), and Japanese Patent Application Publication No. 2006-300206 (JP-A-2006-300206). Generally, this kind of a friction clutch has a mechanism that allows friction elements to slip such that rotational difference occurs, and is able to be switch-controlled between a released (i.e., disengaged) state and a completely engaged state, as well as controlled to a slip-engaged state. For example, in a lock-up clutch, it is determined whether the clutch is engaged or released based on the vehicle state from a preset relationship aimed at improving fuel efficiency and the like. Lock-up control is started when the vehicle state enters a lock-up region. Moreover, the lock-up control region is able to be made wider, thus enabling fuel efficiency to be improved, by executing slip control (lock-up slip control, flex lock-up control) that enables lock-up operation over a wide running range by providing predetermined slippage in the lock-up clutch based on the vehicle state from the preset relationship.
Here, the amount of heat generated in the friction elements of the friction clutch, as well as the temperature of the friction elements themselves, increase the more slip control is executed. Therefore, the durability of the friction elements may decrease depending on the manner in which slip control is executed. In particular, slip control at take-off (hereinafter referred to as “take-off slip control”), that prevents the engine from racing, or overspeeding, and thus improves fuel efficiency by slip-engaging the lock-up clutch when the vehicle takes off from a standstill, is control from the time that the vehicle takes off, at which time the slip amount of the lock-up clutch is relatively large. Therefore, the thermal load of the lock-up clutch is large, so heat resistance may become even more of an issue.
Therefore, various methods that estimate the amount of heat generated in the friction elements and execute slip control taking this amount of heat into account have been proposed to deal with this issue of reduced durability of the friction elements. For example, JP-A-2006-226333 proposes to improve fuel efficiency by regarding the hydraulic fluid temperature at the start of control as the temperature of the sliding portions of the lock-up clutch, and setting, based on the hydraulic fluid temperature, each of an allowed cumulative heating value and a allowed slip engagement time until slip control is canceled, and a required restart time until slip control is restarted after it has been canceled, and then canceling slip engagement control according to the allowed slip engagement time and the allowed cumulative heating value, as well as suppressing the slip engagement control from being canceled more than is necessary while preventing the lock-up clutch from overheating, by restarting the slip engagement control according to the required restart time.
The required restart time described in JP-A-2006-226333 is set to the time that it will take for the temperature to fall from a highest allowable temperature to a restart allowed temperature. That is, a time that allows for the worst cooling from the worst state (i.e., a state with the maximum heat value conceivable; the highest conceivable temperature) (that is, the longest period of time assumed for the temperature to fall to the restart allowed temperature) is set. However, slip control is not limited to always being canceled after the worst state is realized when slip control is executed. When the required restart time is set as described above, slip control may be unable to be restarted even in a state in which slip control would normally be able to be restarted. In this case, slip control is executed less frequently, so there may be fewer opportunities to benefit from the effects of executing slip control. This issue is not well-known, and there is room for further improvement to simultaneously suppress a decrease in the durability of the friction clutch and better obtain the effects from executing slip control.